Aktion T4 (German, pronounced [akˈtsi̯oːn teː fiːɐ]) was a campaign of mass murder by involuntary euthanasia in Nazi Germany.[b] The term was first used in post-war trials against doctors who had been involved in the killings. The name T4 is an abbreviation of Tiergartenstraße 4, a street address of the Chancellery department set up in early 1940, in the Berlin borough of Tiergarten, which recruited and paid personnel associated with Aktion T4.[c] Certain German physicians were authorised to select patients „deemed incurably sick, after most critical medical examination“ and then administer to them a „mercy death“ (Gnadentod). In October 1939, Adolf Hitler signed a „euthanasia note“, backdated to 1 September 1939, which authorised his physician Karl Brandt and Reichsleiter Philipp Bouhler to begin the killing.